1. Field of the Invention
The present invention relates to wireless communication and smart antennas. More specifically, the present invention relates to smart antennas for wireless local area network (“WLAN”), Wi-Fi, and pico-cellular wireless communications systems, including IEEE 802.11 systems. In particular, the present invention provides an innovative Yagi antenna array, which is controllable, and has particular utility as a wired, controllable antenna array for multiple-input and multiple-output (MIMO) telecommunications systems.
2. Description of the Related Art
As is known, a Yagi antenna is a directional antenna having a driven element (typically a dipole or folded dipole) and additional parasitic elements (usually a reflector and one or more directors). The Yagi design operates on the basis of electromagnetic interaction between the parasitic elements and the driven element. The reflector element is typically slightly longer than the driven element, whereas the directors are typically somewhat shorter. This design achieves a substantial increase in the antenna's directionality and gain compared to a simple dipole. See for example U.S. Pat. No. 6,326,922, incorporated herein by reference. Such Yagi antennas are often referred to as beam antennas due to their high gain over a narrow bandwidth, making them useful in various telecommunications systems. However, the beam is fixed due to the linear geometry of the driven element, the reflector, and the director(s).
Means for switching the directionality of Yagi antennas is disclosed in U.S. Pat. No. 7,602,340, incorporated herein by reference. FIG. 46 depicts a structure by which the antenna beam can be switched by 180 degrees. When a positive voltage is applied to the parasitic elements 101, one of them is brought into conduction with the auxiliary elements 103 provided at the respective ends thereof, to thus act as a reflector. The remaining parasitic element 101 is not brought into conduction with the auxiliary elements 103, to thus act as a director. Therefore, the antenna exhibits directivity in the direction of the parasitic element 101 that remains out of conduction with the auxiliary elements 103. When a positive voltage is applied to the parasitic elements 101, the opposite occurs and the beam is switched by 180 degrees. In FIGS. 1 and 2, the first ground conductor 5 and the parasitic element 6 are provided co-planar with the radiating element 3. The switches 7 are short-circuited by means of a control signal output from the control circuit 10, to bring the first ground conductor 5 and the parasitic element 6 into electrical conduction with each other. That is, the radiating element 3 is enclosed by the ground conductor, as shown in (2) of FIG. 2(a). As shown in (2) of FIG. 2(b), the antenna thus exhibits directivity where the maximum radiation arises in directions ±Z. However, when the switches 7 are opened by the control signal output from the control circuit 10; i.e., when a portion surrounding the radiating element 3 is separated from the ground conductor as shown in (3) of FIG. 2(a), the parasitic element 6 acts as a director. As shown in (3) of FIG. 2(b), the antenna becomes unidirectional and exhibits the maximum radiation in a direction +X. Thus, the directivity of the antenna can be 20 switched through about 90 degrees by means of short-circuiting or opening the switches 7. A problem with these approaches is that complicated switching circuitry is required, and antenna beam steering by only 90 degree increments is achieved.
Another useful antenna array for telecommunications is disclosed in U.S. patent application No. 13/871,394, filed Apr. 26, 2013 for “MULTI-BEAM SMART ANTENNA FOR WLAN AND PICO CELLULAR APPLICATIONS”, also incorporated herein by reference.
With the proliferation of wireless local area networks or WLANs, there has been an increase in requirements to find cost effective means to deploy small, efficient access points having MIMO capabilities. In such systems, plural differently-oriented Yagi antennas would enable multi-directional coverage, but would require very many Yagi antennas to cover a wide (e.g., 360 degree) field. Additionally, since each reflector is longer than the driven element, such a multi-Yagi array would have a very large footprint.
The present invention provides method and apparatus to enable a Yagi antenna array to compress the side(s) of reflectors, so that multiple Yagi antennas can be compactly integrated into a single array of elements. The present invention additionally improves the bandwidth of the antenna to enable good return loss across the entire 5 GHz band. Further, the present invention provides unique Yagi and non-Yagi antenna arrays.